The invention relates to adhering a substrate to a surface such as skin using a pressure sensitive adhesive.
Pressure Sensitive Adhesives (PSAs) are normally tacky at room temperature and typically can form a bond to a surface by, at most, light finger pressure. Pressure sensitive adhesive tapes have been used for a variety of marking, holding, protecting, sealing and masking purposes. PSA tapes have many uses in medical applications as well. Such applications typically involve adhering the tape to skin. The irregular and complex surface of the skin presents obstacles in itself, and the wide variation in the skin surface from individual to individual and from one position on the individual to another position compounds these obstacles.
In a first aspect, the invention features an article that includes a substrate having a surface, at least a portion of which is provided with a pressure sensitive adhesive composition that includes a blend of discrete, crosslinked polymer microspheres and a polymer matrix. The adhesive composition has a substantially smooth, exposed surface available for adhesion.
The microspheres can be tacky microspheres, solid microspheres, hollow microspheres, tack-free microspheres or plastic microspheres. Preferred microspheres comprise tacky, hollow microspheres. In a preferred embodiment, the adhesive composition includes between about 1% and about 75% percent by volume of the microspheres.
The microspheres preferably have an average diameter between about 1 micrometer and about 300 micrometers. A preferred matrix polymer includes an acrylic polymer.
In a preferred embodiment, the thickness of the adhesive composition on the substrate is between about 10 micrometers and about 300 micrometers. The article is preferably substantially transparent upon observation by the naked eye. For example, the article is sufficiently transparent such that a health care worker can observe the skin underlying the article.
The adhesive composition can be in the form of a substantially continuous coating on the surface of the substrate or a discontinuous coating on the surface of the substrate. The microspheres preferably include the reaction product of iso-octylacrylate, acrylic acid, and poly(ethylene oxide)acrylate.
In another aspect, the invention features an article adapted for adhesion to the skin of a patient that includes a substrate having a surface, at least a portion of which is provided with a pressure sensitive adhesive composition that includes a blend of discrete, crosslinked polymer microspheres and a polymer matrix. The adhesive composition has a substantially smooth, exposed surface available for adhesion. The article may be provided, e.g., in the form of a skin patch, wound dressing, adhesive bandage, or island dressing.
In a third aspect, the invention features a method of making an article including the steps of:
(a) preparing a pressure sensitive adhesive composition including a blend of discrete, crosslinked polymer microspheres and a polymer matrix; and
(b) depositing the blend on at least a portion of a substrate in the form of a coating,
the average microsphere diameter, the volume fraction of the microspheres in the composition and the thickness of the coating being selected such that the coating has a substantially smooth, exposed surface available for adhesion.
The smooth surface of the adhesives of the present invention provides for more extensive contact with an opposing surface than corresponding adhesives with protruding microspheres. As a result, the initial peel adhesion of the adhesive composition generally is relatively high. Also, the adhesive composition does not exhibit unacceptably high adhesion build-up over time when adhered to an opposing surface. The refractive indices of the polymer microspheres and matrix are generally substantially the same, making it possible to prepare a substantially transparent article. Such articles, in turn, permit observation of the substrate to which the article is adhered. This feature is particularly useful in medical applications because it enables examination of the underlying skin.
Other advantages and features of the invention will be apparent from the detailed description and from the claims.
The invention is directed to articles featuring a substrate coated with a pressure sensitive adhesive composition that includes a polymer matrix blended with discrete, crosslinked polymer microspheres. The average microsphere diameter, volume fraction of microspheres and coating thickness are selected such that when the adhesive composition is applied to the substrate, the adhesive composition forms an exposed surface available for adhesion that is substantially smooth on a scale on the order of the size of the microspheres.
Smoothness of the exposed surface on this order indicates that the microspheres are not protruding from the plane of the surface. This is in contrast to adhesives where protrusion of polymer microspheres, whether or not covered by polymer matrix, provides for positionability. Positionability of adhesives with protruding microspheres is due, at least partly, to more limited point contact with the protruding regions during adhesion to an opposing surface.
The substrate generally can be made from any material suitable for the particular application envisioned for the article. Preferred substrates exhibit a desired combination of properties such as moisture vapor permeability, texture, conformability, yield modulus, appearance, processability, and strength. A substrate can have structure on its surface as long as the structure does not interfere with the formation of a smooth layer of adhesive at an appropriate adhesive thickness. For certain applications (e.g., transparent dressings), it is preferred for the substrates to be substantially transparent upon observation by the naked eye of an observer.
Suitable materials for flexible substrates include paper, latex saturated paper, polymeric film, metallic foil, and ceramic sheeting. Appropriate materials for polymeric films include cellulose acetate film, ethyl cellulose film, polyolefins (such as polyethylene and polypropylene, including isotactic polypropylene), polystyrene, polyvinyl alcohol, polyester (e.g., poly(ethylene terephthalate) or poly(butylene terephthalate)), poly(caprolactam), poly(vinylidene fluoride), and the like. Suitable substrates also include commercially available fabrics such as non-woven, woven or knitted fabrics. Such fabrics may be constructed from a wide range of synthetic or natural fibers, used singly or in blends. Examples of suitable non-woven fabrics include carded, spun-bonded, spun-laced, air-laid, blown microfibrous constructions, and stitch-bonded fabrics.
Suitable commercially available substrate materials include kraft paper (available from Monadnock Paper, Inc.); cellophane (available from Flexel Corp.); spun-bond poly(ethylene) and polypropylene, such as Tyvek(trademark) and Typar(trademark) (available from DuPont, Inc.); and porous films obtained from polyethylene and poly(propylene), such as Teslin(trademark) (available from PPG Industries, Inc.), and Cellguard(trademark) (available from Hoechst-Celanese).
Release coated substrates can also be used. Such substrates are typically employed when an adhesive transfer tape is provided. Examples of release coated substrates include silicone coated kraft paper and the like. Tapes of the invention may also incorporate a low adhesion backsize (LAB). The LAB typically is applied to the substrate surface that is opposite the surface bearing the pressure sensitive adhesive.
The adhesive compositions of the present invention are particularly suitable for the production of medical articles intended for adhesion to skin. Examples include tapes, skin patches, strips, wound dressings, monitoring or neuro-stimulating electrodes, transparent adhesive dressings, island dressings (with absorbent polymeric or fabric islands), consumer first aid dressings, drapes, and the like. Suitable substrates for these applications include conformable backing materials that are known in the medical or surgical fields. Useful substrates include nonwoven fabrics, woven fabrics, knit fabrics, and low to medium tensile modulus synthetic films such as polypropylene, polyethylene, polyvinyl chloride, polyurethane, low modulus polyester and ethyl cellulose. Fabrics can be made from materials such as cotton, nylon, rayon or other natural or synthetic fibers or blends. The films preferably have a tensile modulus less than about 400,000 psi as measured in accordance with ASTM D-638 and D-882 procedures, preferably less than about 300,000 psi.
The desirable features of the adhesives for these applications include relatively high initial peel adhesion, minimal adhesion build-up over time and, optionally, transparency. Preferred articles have moisture vapor transmission rates, when tested in accordance with ASTM E-96-80, of at least about 500 g/m2, over 24 hours at 38xc2x0 C., with a humidity differential of 80 percent, more preferably 1000 g/m2.
In addition, it has been found that the higher the creep compliance, the greater the quantity of adhesive residue left on the skin after removal of the adhesive coated article. Creep compliance is a rheological property relating to the flow of the adhesive. Accordingly, creep compliance values less than 2.3xc3x9710xe2x88x925 cm2/dyne are preferred. Measurement of creep compliance values is described below.
Preferred substrates have a high rate of moisture vapor transmission. For example, a continuous film substrate of 25 xcexcm thickness prepared from a polyurethane sold under the tradename Estane 58309, available from B. F. Goodrich, and a continuous film substrate prepared from a polyester sold under the tradename Hytrel 4053, available from DuPont, each have moisture vapor transmission values of about 1000 to about 1500 g/m2/24 hours. Woven substrates such as those used for DURAPORE(trademark) tape, available from 3M, have even higher values.
The adhesive compositions will now be described in greater detail. All amounts are in weight percent unless otherwise noted.
Polymer Matrix
The polymer matrix preferably is a pressure sensitive adhesive. It can be formed from a variety of materials. Suitable materials for the matrix include rubber resin polymers, including natural or synthetic rubber and block copolymers, and free radically polymerizable acrylic pressure sensitive adhesive compositions. The acrylic adhesives are less prone to discoloration and are amenable to precise control during preparation.
The acrylate monomers are typically alkyl acrylates, preferably monofunctional unsaturated acrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which have from 2 to about 14 carbon atoms, providing a polymer having a glass transition temperature (Tg) of less than 0xc2x0 C., preferably less than xe2x88x9210xc2x0 C. Included within this class of preferred monomers are, for example, iso-octyl acrylate, iso-nonyl acrylate, 2-ethyl-hexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl acrylate, hexyl acrylate, and mixtures thereof.
The alkyl acrylate monomers can be used to form homopolymers, or they can be copolymerized with polar copolymerizable monomers or higher Tg monomers (higher than the alkyl acrylate) such as some vinyl esters, and C1 to C4 alkyl esters of (meth)acrylic acid and/or styrene. When copolymerized with polar monomers, the alkyl acrylate monomer generally comprises at least about 70% of the polymerizable monomer composition. A portion of high Tg monomers can be used as long as the Tg of the resulting copolymer is less than about 10xc2x0 C.
The polar copolymerizable monomers can be selected from monomers such as monoolefinic mono- and dicarboxylic acids, hydroxyalkyl acrylates, cyanoalkyl acrylates, acrylamides or substituted acrylamides, N-vinyl pyrrolidone, acrylonitrile, vinyl chloride and diallyl phthalate. The polar monomer preferably comprises up to about 25%, more preferably up to about 15%, of the polymerizable monomer composition.
Optionally, a low molecular weight hydrophobic polymer can be added to the adhesive matrix monomers to improve emulsion stability. These polymers preferably have an average molecular weight from 400 to 50,000 and include polystyrene resins, poly(methylmethacrylate) resin, polybutadiene, polyisoprene, poly(alphamethylstyrene), polydiene-polyaromatic arene copolymers, rosin esters and mixtures thereof. These may be added in amounts up to 20% of the monomer mixture, preferably up to 10%.
Also usable are copolymerizable ionic surfactants to improve cohesive strength and moisture resistance. These include polyalkylene polyalkoxy ammonium sulfate (e.g., xe2x80x9cMAZONxe2x80x9d SAM-211 available from PPG Industries) and alkyl allyl sulfosuccinates (e.g., xe2x80x9cTREMxe2x80x9d LF40 available from Diamond Shamrock Co.) as well as those described in PCT application No. WO 89/12618 and U.S. Pat. Nos. 3,925,442 and 3,983,166. Non-copolymerizable ionic and nonionic surfactants can be used instead of the copolymerizable surfactants but are less preferred. The surfactants can be used in amounts of from 0 to 10% of the total monomer component, preferably 1.5 to 5%.
The pressure sensitive adhesive matrix is prepared from a polymerizable composition preferably containing initiator to aid in polymerization of the monomers. Suitable initiators include thermally-activated initiators where the initiator is water or oil soluble. Suitable oil soluble initiators include azo and diazo compounds, hydroperoxides, peroxides, and the like. Water soluble initiators include persulfates such as potassium persulfate. Generally, the initiator is present in an amount from about 0.01% to about 3.0%, preferably 0.1 to 0.5%, based on the total monomer component.
Where superior cohesive strengths are desired, the pressure sensitive adhesive matrix may also be cross-linked. Preferred crosslinking agents for the acrylic pressure-sensitive adhesive matrix are multiacrylates such as 1,6-hexanediol diacrylate, as well as those disclosed in U.S. Pat. No. 4,379,201 (Heilmann et al.), incorporated herein by reference. Photo-initiators can act as post-cure crosslinkers. Examples include the benzoin ethers, substituted benzoin ethers such as benzoin methyl ether or benzoin iso-propyl ether, substituted acetophenones such as 2,2-diethoxy-acetophenone, and 2,2-dimethoxy-2-phenyl-acetophenone, substituted alpha-ketols such as 2-methyl-2-hydroxypropiophenone, aromatic sulphonyl chlorides such as 2-naphthalene sulphonyl chloride, and photoactive oximes such as 1-phenyl-1,1-propanedione-2-(O-ethoxycarbonyl)oxime. Each of the crosslinking agents is useful in the range of from about 0.01% to about 3%, preferably 0.1 to 1%, of the total components.
Other useful materials that can be blended into the adhesive matrix include, but are not limited to, fillers, pigments, plasticizers, tackifiers, fibrous reinforcing agents, woven and nonwoven fabrics, foaming agents, antioxidants, stabilizers, fire retardants, and rheological modifiers. Chain transfer agents, such as carbon tetrabromide, mercaptans or alcohols, can be used in the monomer mixture to adjust the molecular weight of the resulting polymer.
Microspheres
The polymer microspheres are crosslinked. In addition, they can be solid or hollow and tacky or tack-free. Tack-free microspheres can be elastomeric or plastic. The specific type of microsphere can be selected to yield the desired properties of the adhesive composition for the particular application. The microspheres should be water and solvent insoluble, but solvent dispersible. Furthermore, the microspheres may be swellable in organic solvents. Polymer microspheres preferably are formed by free radical suspension polymerization.
The diameter of the individual microspheres preferably is selected such that the adhesive forms a smooth surface for a given microsphere volume fraction and coating thickness. The microspheres generally will have an average diameter between about 1 micrometer (xcexcm) and 300 xcexcm, more preferably between 5 xcexcm and 100 xcexcm and even more preferably between 10 xcexcm and 70 xcexcm. When the microspheres are hollow, the voids typically range in size from less than 1 xcexcm up to about 100 xcexcm or larger.
For the formation of tacky or tack-free, elastomeric microspheres, preferred monomers include vinyl esters, acrylates and methacrylates, alone or in combination with each other such that the Tg of the polymer is less than about room temperature. Combinations of monomers that result in a Tg greater than room temperature will result in plastic, tack-free microspheres. Examples of appropriate monomers for the formation of elastomeric microspheres include iso-octyl acrylate, iso-nonyl acryiate, iso-amyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate, sec-butyl acrylate, tert-butyl acrylate, iso-bornyl acrylate, butyl methacrylate, vinyl acetate, acrylonitrile, iso-decyl acrylate, iso-decyl methacrylate, 2-methylbutyl acrylate, 4-methyl-2-pentyl acrylate, and ethyl acrylate.
Examples of suitable vinyl ester monomers include vinyl 2-ethylhexanoate, vinyl caprate, vinyl laurate, vinyl pelargonate, vinyl hexanoate, vinyl propionate, vinyl decanoate, vinyl octanoate, and other monofunctional unsaturated vinyl esters of linear or branched carboxylic acids comprising 1 to 14 carbon atoms. Preferred vinyl ester monomers include vinyl laurate, vinyl caprate, vinyl-2-ethylhexanoate, and mixtures thereof.
The vinyl esters, acrylates or methacrylates may be copolymerized with other vinyl monomers including styrene, substituted styrenes, vinyl benzene, N-iso-octylacrylamide, vinyl chloride and vinylidene chloride. Minor amounts of other comonomers known in the art can be employed, provided that the Tg of the resulting copolymer stays within the desired range.
For the formation of plastic microspheres, free radically polymerizable monomers are selected that are capable of forming homo- or co-polymers having glass transition temperatures generally above 20xc2x0 C. Suitable monomers or comonomers include vinyl esters, alkyl acrylates, alkyl methacrylates, styrenes and substituted styrenes, cyclic alkyl acrylates and methacrylates, aryl acrylates and methacrylates and mixtures thereof. Suitable vinyl esters include vinyl neonanoate, vinyl pivalic acid ester, vinyl acetate, vinyl propionate, and vinyl neodecanoate. Acrylates and methacrylates can be used provided that they do not cause the resultant polymer to have a Tg or Tm of less than about 10xc2x0 C. For plastic microspheres, preferred are acrylates and methacrylates which will produce homopolymers or copolymers having Tg higher than about 0xc2x0 C. and preferably higher than about 10xc2x0 C. Suitable acrylates and methacrylates include tert-butyl acrylate, iso-bornyl acrylate, butyl methacrylate, vinyl acetate, acrylonitrile, iso-nonal acrylate, iso-decyl acrylate, iso-decyl methacrylate, sec-butyl acrylate, iso-amyl acrylate, 2-methylbutyl acrylate, 4-methyl-2-pentyl acrylate, iso-decyl acrylate, ethyl acrylate and mixtures thereof. Suitable acrylates can be copolymerized with vinyl esters and other suitable comonomers.
Also useful as comonomers are other vinyl monomers such as vinyl benzene, divinyl benzene, N-iso-octylacrylamide, which can be used in conjunction with the vinyl ester, acrylate, methacrylate or acrylic monomers. Minor amounts of other comonomers known in the art can be employed, provided that the Tg of the comonomer stays within the desired range.
For the production of either elastomeric or plastic microspheres, other suitable co-monomers include polar co-monomers, e.g., monoolefinic monocarboxylic acids, monoolefinic dicarboxylic acids, acrylamides, N-substituted acrylamides, salts thereof, and mixtures thereof. Specific examples include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, sulfoethyl methacrylate, and ionic monomers such as sodium methacrylate, ammonium acrylate, sodium acrylate, trimethylamine p-vinyl benzimide, 4,4,9-trimethyl-4-azonia-7-oxo-8-oxa-dec-9-ene-1-sulphonate, N,N-dimethyl-N-(beta-methacryloxy-ethyl) ammonium propionate betaine, trimethylamine methacrylimide, 1,1-dimethyl-l-(2,3-dihydroxypropyl)amine methacrylimide, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylamide, t-butyl acrylamide, dimethyl amino ethyl acrylamide, N-octyl acrylamide, mixtures thereof, and the like. Preferred polar monomers include monoolefinic monocarboxylic acids, monoolefinic dicarboxylic acids, acrylamides, N-substituted acrylamides, salts thereof and mixtures thereof. Examples of such monomers include but are not limited to acrylic acid, sodium acrylate, N-vinyl pyrrolidone, and mixtures thereof.
Hydrophilizing agents or components can also be used as co-monomers to produce microspheres with pendent hydrophilic moieties. The hydrophilizing agents can act as crosslinkers when they are multi-functional. Preferred are free radically reactive hydrophilic oligomers (a polymer having a low number of repeating units, generally 2 to 20) and/or polymers including poly(alkylene oxides) (e.g., poly(ethylene oxide)), poly(vinyl methyl ether), poly(acrylamide), poly(N-vinylpyrrolidone), poly(vinyl alcohol), cellulose derivatives and mixtures thereof.
Other suitable hydrophilizing co-monomers include macromonomers, e.g., acrylate terminated poly(ethylene oxide), methacrylate terminated poly(ethylene oxide), methoxy poly(ethylene oxide) methacrylate, butoxy poly(ethylene oxide) methacrylate, p-vinyl benzyl terminated poly(ethylene oxide), acrylate terminated poly(ethylene glycol), methacrylate terminated poly(ethylene glycol), methoxy poly(ethylene glycol) methacrylate, butoxy poly(ethylene glycol) methacrylate, p-vinyl benzyl terminated poly(ethylene glycol), poly(ethylene oxide) diacrylate, poly(ethylene oxide) dimethacrylate, and mixtures thereof. These functionalized materials are preferred because they are easily prepared through well-known ionic polymerization techniques and are also highly effective in providing grafted hydrophilic segments along free radically polymerized microsphere polymer backbones.
Other examples of suitable macromonomers include p-vinyl benzyl terminated poly(N-vinyl pyrrolidone), p-vinyl benzyl terminated poly(acrylamide), methacrylate terminated poly(N-vinyl pyrrolidone), and mixtures thereof. These macromonomers may be prepared through the esterification reaction of a carboxy terminated N-vinyl pyrrolidone or acrylamide, beta-mercaptopropionic acid chain transfer agent, and chloromethyl styrene or methacryloyl chloride as described in a series of papers by M. Acacia et al. [Angew, Makromol, Chem., 132, 81 (1985); J. Appl. Polym. Sci., 39, 2027 (1990); J. Polym. Sci., Part A: Polym. Chem., 27, 3521 (1989)].
The elastomeric microspheres preferably comprise at least about 70 parts of at least one free radically polymerizable monomer, optionally up to about 30 parts of one or more polar monomers, and about 0 to about 30 parts of at least one hydrophilizing component.
More preferably, the elastomeric microspheres comprise about 80 to about 100 parts, most preferably 90 to 100 parts, of one or more free radically polymerizable monomers selected from the group consisting of alkyl acrylate esters, alkyl methacrylate esters, vinyl esters, and mixtures thereof where the alkyl group is a C4 to C12 alkyl, optionally up to about 10 parts of at least one polar monomer, and optionally up to about 10 parts of a hydrophilizing component. Most preferably the microspheres comprise about 95 to about 99.9 parts of the free radically polymerizable monomers, up to about 5.0 parts of a hydrophilizing component, and, optionally, about 0.1 to about 5.0 parts of a polar monomer.
The composition from which the elastomeric or plastic microspheres of the invention are made may also contain a multifunctional crosslinking agent. The term xe2x80x9cmultifunctionalxe2x80x9d as used herein refers to crosslinking agents which possess two or more free radically polymerizable ethylenically unsaturated groups. Useful multifunctional crosslinking agents include acrylic or methacrylic esters of diols such as butanediol diacrylate, triols such as glycerol, and tetraols such as pentaerythritol. Other useful crosslinking agents include polymeric multifunctional (meth)acrylates, e.g., poly(ethylene oxide) diacrylate or poly(ethylene) oxide dimethacrylate; polyvinylic crosslinking agents, such as substituted and unsubstituted divinylbenzene; and difunctional urethane acrylates, such as xe2x80x9cEBECRYLxe2x80x9d 270 and xe2x80x9cEBECRYLxe2x80x9d 230 (1500 weight average molecular weight and 5000 weight average molecular weight acrylated urethanes, respectivelyxe2x80x94both available from Radcure Specialties), and mixtures thereof.
When a crosslinker is employed, it is typically employed at a level of up to about 10 equivalent weight percent. Above about 0.15 equivalent weight percent, based on the total polymerizable microsphere composition, most elastomeric microspheres become tack-free. The xe2x80x9cequivalent weight percentxe2x80x9d of a given compound is defined as the number of equivalents of that compound divided by the total number of equivalents in the total (microsphere) composition, where an equivalent is the number of grams divided by the equivalent weight. The equivalent weight is defined as the molecular weight divided by the number of polymerizable groups in the monomer (in the case of those monomers with only one polymerizable group, equivalent weight=molecular weight). The crosslinker can be added at any time before 100% conversion to polymer of the monomers of the microsphere composition. Preferably, crosslinker is added before initiation occurs.
The relative amounts of the components are important to the properties of the resultant microspheres. Generally, the greater the amount of crosslinker the less tack in the resulting microspheres. Tacky microspheres generally include crosslinkers up to concentrations where the crosslinkers contribute about 0.15% of the total polymerizable functional groups.
The plastic microspheres preferably comprise at least about 80 parts of at least one free radically polymerizable monomer, optionally up to about 5 parts of one or more polar monomers, about 0 to about 15 parts of at least one hydrophilizing component crosslinked with at least one multifunctional crosslinker. An additional initiator and/or other multifunctional crosslinker and other additives may also be used. More preferably, the microspheres include about 95 to about 100 parts of free radically polymerizable monomer selected from the group consisting of alkyl acrylate esters, alkyl methacrylate esters, vinyl esters, and mixtures thereof, optionally about 0 to about 3 parts of at least one polar monomer, and optionally about 0 to about 2 parts of a hydrophilizing component.
If hollow, elastomeric or plastic microspheres are desired, they may be obtained via a xe2x80x9ctwo-stepxe2x80x9d process comprising the steps of:
(a) forming a water-in-oil emulsion by mixing (1) an aqueous solution (which may contain some of the carbonyl monomer and/or some of the optional polar monomer) with (2) oil phase base monomers, a free radical polymerization initiator, and internal crosslinking agent (if any is used);
(b) forming a water-in-oil-in-water emulsion by dispersing the water-in-oil emulsion from step
(a) into an aqueous phase (containing any of the carbonyl monomer and/or polar monomer not added in step (a)); and
(c) initiating suspension polymerization, usually by applying heat (preferably about 40 to 60xc2x0 C., more preferably about 50 to 60xc2x0 C.) or radiation (e.g., ultraviolet radiation).
Emulsifiers having a low hydrophilic-lipophilic balance (HLB) value are used to facilitate the formation (usually by agitation) of the water-in-oil emulsion in the first step. Suitable emulsifiers are those having an HLB value below about 7, preferably in the range of about 2 to 7. Examples of such emulsifiers include sorbitan monooleate, sorbitan trioleate, and ethoxylated oleyl alcohol such as Brij(trademark) 93, available from Atlas Chemical Industries, Inc. A thickening agent, e.g., methyl cellulose, may also be included in the aqueous phase of the water-in-oil emulsion.
The aqueous phase into which the water-in-oil emulsion is dispersed in step (b) contains an emulsifier having an HLB value above about 7. Examples of such emulsifiers include ethoxylated sorbitan monooleate, ethoxylated lauryl alcohol, and alkyl sulfates. The emulsifier concentration (for both steps (a) and (b)) should be greater than its critical micelle concentration, which refers to the minimum concentration of emulsifier necessary for the formation of micelles, i.e., submicroscopic aggregations of emulsifier molecules. Critical micelle concentration is slightly different for each emulsifier, usable concentrations ranging from about 1.0xc3x9710xe2x88x924 to about 3.0 moles/liter. Additional detail concerning the preparation of water-in-oil-in-water emulsions, i.e. multiple emulsions, may be found in various literature references, e.g., Surfactant Systems: Their Chemistry, Pharmacy, and Biology, (D. Attwood and A. T. Florence, Chapman and Hall Limited, New York, 1983).
Useful initiators are those which are normally suitable for free radical polymerization of acrylate or vinyl ester monomers and which are oil soluble and of very low solubility in water, typically less than 1 g/100 g water at 20xc2x0 C. Examples of such initiators include azo compounds, hydroperoxides, peroxides, and the like, and photoinitiators such as benzophenone, benzoin ethyl ether, 2,2-dimethoxy-2-phenyl acetophenone. The initiator is generally used in an amount ranging from about 0.01% up to about 10% by weight of the total polymerizable composition, preferably up to about Use of a substantially water soluble polymerization initiator, such as those generally used in emulsion polymerizations, causes formation of substantial amounts of latex. During suspension polymerization, any significant formation of latex is undesirable because of the extremely small particle size.
Hollow microspheres may also be prepared by a simpler xe2x80x9cone-stepxe2x80x9d process comprising aqueous suspension polymerization of the carbonyl monomer, the base monomer, and the polar monomer (which is not optional for this process) in the presence of an emulsifier which is capable of producing, inside the droplets, a water-in-oil emulsion that is substantially stable during both formation of the emulsion and subsequent suspension polymerization.
Useful emulsifiers are anionic materials having an HLB value greater than 25 and include alkylaryl ether sulfates such as sodium alkylaryl ether sulfate, e.g., Triton(trademark) W/30, available from Rohm and Haas; alkylaryl poly(ether) sulfates such as alkylaryl poly(ethylene oxide) sulfates, preferably those having up to about 4 ethoxy repeat units; and alkyl sulfates, such as sodium lauryl sulfate, and sodium hexadecyl sulfate, triethanolamine lauryl sulfate, and sodium hexadecyl sulfate; alkyl poly(ether) sulfates, such as alkyl poly(ethylene oxide) sulfates, preferably those having up to about 4 ethoxy units. Alkyl sulfates, alkyl ether sulfates, alkylaryl ether sulfates, and mixtures thereof are preferred.
Non-ionic emulsifiers having an HLB value of between about 13 and 25 can be utilized in conjunction with the anionic emulsifiers. Examples of non-ionic emulsifiers include Siponic(trademark) Y-500-70 (ethoxylated oleyl alcohol, available from Alcolac, Inc.), PLURONIC(copyright) P103, and Tween(trademark)xe2x80x9440 (from ICI America). As in the two-step process, the emulsifier is utilized in a concentration greater than its critical micelle concentration. Polymeric stabilizers may also be present but are not necessary.
The above-described one-step method may be varied by combining the base monomer with non-ionic emulsifiers, oil soluble polymerization initiator, and any multifunctional internal crosslinker before the base monomer is added to the aqueous phase containing a carbonyl monomer, emulsifier and any optional polar monomer. (The polar monomer is optional for this process.) The resulting emulsion is suspension polymerized to yield hollow pressure sensitive adhesive microspheres. Anionic emulsifiers with an HLB value greater than 7 may be included in the aqueous phase to stabilize the system during suspension polymerization but are not required.
Solid pressure sensitive adhesive microspheres may be prepared via the suspension polymerizations disclosed in U.S. Pat. Nos. 3,691,140; 4,166,152. In general, these suspension polymerization techniques use ionic or non-ionic emulsifiers in an amount greater than the critical micelle concentration and/or protective colloids, finely divided inorganic solids, or the like.
Each suspension polymerization method (whether producing hollow or solid microspheres) may be modified by withholding the addition of all or some of the carbonyl monomer and/or any optional polar monomer until after polymerization of the oil phase base monomer has been initiated. In this instance, however, these components must be added to the polymerizing mixture prior to 100% conversion of the base monomer. Similarly, the internal crosslinker (if used) can be added at any time before 100% conversion to polymer of the monomers of the microsphere composition. Preferably it is added before initiation occurs. The hydrophilizing component can be added to the oil or water phase in the first step or the water phase in the second step, either before or after polymerization is initiated, or some combination of these options.
Following polymerization, an aqueous suspension of the hollow or solid microspheres is obtained which is stable to agglomeration or coagulation under room temperature conditions (i.e., about 20 to about 25xc2x0 C.). The suspension may have a non-volatile solids content of from about 10 to about 60 percent by weight.
The pressure sensitive adhesive properties of the microspheres may be altered by the addition of tackifying resin and/or plasticizer. Other components, such as pigments, neutralizing agents such as sodium hydroxide, etc., fillers, stabilizers, chain transfer agents, and various polymeric additives may be included as well.
Preparation of the Adhesive Article
The adhesive composition is preferably prepared by blending the polymer matrix with the appropriate quantity of microsphere suspension. The resulting blend is then coated onto a substrate using standard techniques. Alternatively, the blend may be prepared by combining the microspheres with polymerizable monomers and/or oligomers, coating the resulting mixture onto a backing, and then exposing the entire article to an energy source (e.g., heat, ultraviolet radiation, or ionizing radiation) to polymerize the monomers and/or oligomers, thereby forming the polymer matrix. This technique is described generally in Delgado et al., U.S. Pat. No. 5,266,402. The average microsphere diameter, volume fraction of microspheres, and coating thickness are selected such that the adhesive composition forms a substantially smooth surface after being applied to the substrate. The adhesive coating can cover the entire surface of the substrate or only a portion of the surface. Furthermore, the coating can be continuous or discontinuous (e.g., in the form of a dot or grid pattern). For a discussion of discontinuous coatings, see for example U.S. Pat. Nos. 4,595,001 and 4,798,201.